libinput and wheel tilt events

We're in the middle of the 1.7 development cycle and one of the features merged already is support for "wheel tilt", i.e. support for devices that don't have a separate horizontal wheel but instead rely on a tilt motion for horizontal event. Now, the way this is handled in the kernel is that the events are sent via REL_WHEEL (or REL_DIAL) so we don't actually need special code in libinput to handle tilt. But libinput tries to to make sense of input devices so the upper layers have a reliable base to build on - and that's why we need tilt-wheels to be handled.

For 'pointer axis' events (i.e. scroll events) libinput provides scroll sources. These specify how the scroll event was generated, allowing a caller to handle things accordingly. A finger-based scroll for example can trigger kinetic scrolling while a mouse wheel would not usually do so. The value for a pointer axis is also dependent on the scroll source - for continuous/finger based scrolling the value is in pixels. For a mouse wheel, the value is in degrees. This obviously doesn't work for a tilt event because degrees don't make sense in this context. So the new axis source is just that, an indicator that the event was caused by a wheel tilt rather than a rotation. Its value matches the default wheel rotation (i.e. 15 degrees) just to make use of it easier.

Of course, a device won't tell us whether it provides a proper wheel or just tilt. So we need a hwdb property and I've added that to systemd's repo. To make this work, set the MOUSE_WHEEL_TILT_HORIZONTAL and/or MOUSE_WHEEL_TILT_VERTICAL property on your hardware and you're off. Yay.

Patches for the wayland protocol have been merged as well, so this is/will be available to wayland clients.